Method and apparatus for currency discrimination and counting

ABSTRACT

An improved method and apparatus for discriminating between currency bills of different denominations uses an optical sensing and correlation technique based on the sensing of bill reflectance characteristics obtained by illuminating and scanning a bill along its narrow dimension. A series of detected reflectance signals are obtained by sampling and digitally processing, under microprocessor control, the reflected light at a plurality of predefined sample points as a currency bill is moved across an illuminated strip with its narrow dimension parallel to the direction of transport of the bill. The sample data is subjected to digital processing, including a normalizing process, whereby the reflectance data represents a characteristic pattern that is unique for a given bill denomination and incorporates sufficient distinguishing features between characteristic patterns for discriminating between different currency denominations. A plurality of master characteristic patterns are generated and stored using original bills for each denomination of currency to be detected. The pattern generated by scanning a bill under test and processing the data samples is compared with each of the prestored master patterns to generate, for each comparison, a correlation number representing the extent of similarity between corresponding ones of the plurality of data samples for the compared patterns. Denomination identification is based on designating the scanned bill as belonging to the denomination corresponding to the stored master pattern for which the correlation number resulting from pattern comparison is determined to be the highest, subject to a bi-level threshold of correlation.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of pending U.S. patentapplication Ser. No. 07/475,111, filed Feb. 5, 1990, now abandoned, for"Method and Apparatus for Currency Discrimination and Counting."

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates, in general, to currency identification.The invention relates more particularly to a method and apparatus forautomatic discrimination and counting of currency bills of differentdenominations using light reflectivity characteristics of indiciaprinted upon the currency bills.

2. Description of the Related Art

A variety of techniques and apparatus have been used to satisfy therequirements of automated currency handling systems. At the lower end ofsophistication in this area of technology are systems capable ofhandling only a specific type of currency, such as a specific dollardenomination, while rejecting all other currency types. At the upper endare complex systems which are capable of identifying and discriminatingamong and automatically counting multiple currency denominations.

Currency discrimination systems typically employ either magnetic sensingor optical sensing for discriminating between different currencydenominations. Magnetic sensing is based on detecting the presence orabsence of magnetic ink in portions of the printed indicia on thecurrency by using magnetic sensors, usually ferrite core-based sensors,and using the detected magnetic signals, after undergoing analog ordigital processing, as the basis for currency discrimination. The morecommonly used optical sensing technique, on the other hand, is based ondetecting and analyzing variations in light reflectance ortransmissivity characteristics occurring when a currency bill isilluminated and scanned by a strip of focused light. The subsequentcurrency discrimination is based on the comparison of sensed opticalcharacteristics with prestored parameters for different currencydenominations, while accounting for adequate tolerances reflectingdifferences among individual bills of a given denomination.

A major obstacle in implementing automated currency discriminationsystems is obtaining an optimum compromise between the criteria used toadequately define the characteristic pattern for a particular currencydenomination, the time required to analyze test data and compare it topredefined parameters in order to identify the currency bill underscrutiny, and the rate at which successive currency bills may bemechanically fed through and scanned. Even with the use ofmicroprocessors for processing the test data resulting from the scanningof a bill, a finite amount of time is required for acquiring samples andfor the process of comparing the test data to stored parameters toidentify the denomination of the bill.

Most of the optical scanning systems available today utilize complexalgorithms for obtaining a large number of reflectance data samples as acurrency bill is scanned by an optical scanhead and for subsequentlycomparing the data to corresponding stored parameters to identify thebill denomination. Conventional systems require a relatively largenumber of optical samples per bill scan in order to sufficientlydiscriminate between currency denominations, particularly thosedenominations for which the reflectance patterns are not markedlydistinguishable. The use of the large number of data samples slows downthe rate at which incoming bills may be scanned and, more importantly,requires a correspondingly longer period of time to process the data inaccordance with the discrimination algorithm.

A major problem associated with conventional systems is that, in orderto obtain the required large number of reflectance samples required foraccurate currency discrimination, such systems are restricted toscanning bills along the longer dimension of currency bills. Lengthwisescanning, in turn, has several inherent drawbacks including the need foran extended transport path for relaying the bill lengthwise across thescanhead and the added mechanical complexity involved in accommodatingthe extended path as well as the associated means for ensuring uniform,non-overlapping registration of bills with the sensing surface of thescanhead.

The end result is that systems capable of accurate currencydiscrimination are costly, mechanically bulky and complex, and generallyincapable of both currency discrimination and counting at high speedswith a high degree of accuracy.

SUMMARY OF THE INVENTION

It is a principal object of the present invention to provide an improvedmethod and apparatus for identifying and counting currency billscomprising a plurality of currency denominations.

It is another object of this invention to provide an improved method andapparatus of the above kind which is capable of efficientlydiscriminating among and counting bills of several currencydenominations at a high speed and with a high degree of accuracy.

A related object of the present invention is to provide such an improvedcurrency discrimination and counting apparatus which is compact,economical, and has uncomplicated construction and operation.

Briefly, in accordance with the present invention, the objectivesenumerated above are achieved by means of an improved optical sensingand correlation technique adopted to both counting and denominationdiscrimination of currency bills. The technique is based on the opticalsensing of bill reflectance characteristics obtained by illuminating andscanning a bill along its narrow dimension, approximately about thecentral section of the bill. Light reflected from the bill as it isoptically scanned is detected and used as an analog representation ofthe variation in the dark and light content of the printed pattern orindicia on the bill surface.

A series of such detected reflectance signals are obtained by samplingand digitally processing, under microprocessor control, the reflectedlight at a plurality of predefined sample points as the bill is movedacross the illuminated strip with its narrow dimension parallel to thedirection of transport of the bill. Accordingly, a fixed number ofreflectance samples is obtained across the narrow dimension of the note.The data samples obtained for a bill scan are subjected to digitalprocessing, including a normalizing process to deaccentuate variationsdue to "contrast" fluctuations in the printed pattern or indiciaexisting on the surface of the bill being scanned. The normalizedreflectance data represent a characteristic pattern that is fairlyunique for a given bill denomination and incorporates sufficientdistinguishing features between characteristic patterns for differentcurrency denominations so as to accurately differentiate therebetween.

By using the above approach, a series of master characteristic patternsare generated and stored using standard bills for each denomination ofcurrency that is to be detected. The "standard" bills used to generatethe master characteristic patterns are preferably bills that areslightly used bills. According to a preferred embodiment, twocharacteristic patterns are generated and stored within system memoryfor each detectable currency denomination. The stored patternscorrespond, respectively, to optical scans performed on the greensurface of a bill along "forward" and "reverse" directions relative tothe pattern printed on the bill. For bills which produce significantpattern changes when shifted slightly to the left or right, such as the$10 bill in U.S. currency, it is preferred to store two patterns foreach of the "forward" and "reverse directions, each pair of patterns forthe same direction represent two scan areas that are slightly displacedfrom each other along the long dimension of the bill. Preferably, thecurrency discrimination and counting method and apparatus of thisinvention is adapted to identify seven (7) different denominations ofU.S. currency, i.e., $1, $2, $5, $10, $20, $50 and $100. Accordingly, amaster set of 16 different characteristic patterns is stored within thesystem memory for subsequent correlation purposes (four patterns for the$10 bill and two patterns for each of the other denominations.

According to the correlation technique of this invention, the patterngenerated by scanning a bill under test and processing the sampled datais compared with each of the 16 prestored characteristic patterns togenerate, for each comparison, a correlation number representing theextent of similarity between corresponding ones of the plurality of datasamples for the compared patterns. Denomination identification is basedon designating the scanned bill as belonging to the denominationcorresponding to the stored characteristic pattern for which thecorrelation number resulting from pattern comparison is determined to bethe highest. The possibility of a scanned bill having its denominationmischaracterized following the comparison of characteristic patterns, issignificantly reduced by defining a bi-level threshold of correlationthat must be satisfied for a "positive" call to be made.

In essence, the present invention provides an improved optical sensingand correlation technique for positively identifying any of a pluralityof different bill denominations regardless of whether the bill isscanned along the "forward" or "reverse" directions. The invention isparticularly adapted to be implemented with a system programmed to trackeach identified currency denomination so as to conveniently present theaggregate total of bills that have been identified at the end of a scanrun. Also in accordance with this invention, currency detecting andcounting apparatus is disclosed which is particularly adapted for usewith the novel sensing and correlation technique summarized above. Theapparatus incorporates an abbreviated curved transport path foraccepting currency bills that are to be counted and transporting thebills about their narrow dimension across a scanhead located downstreamof the curved path and onto a conventional stacking station where sensedand counted bills are collected. The scanhead operates in conjunctionwith an optical encoder which is adapted to initiate the capture of apredefined number of reflectance data samples when a bill (and, thus,the indicia or pattern printed thereupon) moves across a coherent stripof light focused downwardly of the scanhead.

The scanhead uses a pair of light-emitting diodes ("LED"'s) to focus acoherent light strip of predefined dimensions and having a normalizeddistribution of light intensity across the illuminated area. The LED'sare angularly disposed and focus the desired strip of light onto thenarrow dimension of a bill positioned flat across the scanning surfaceof the scanhead. A photo detector detects light reflected from the bill.The photo detector is controlled by the optical encoder to obtain thedesired reflectance samples.

Initiation of sampling is based upon detection of the change inreflectance value that occurs when the outer border of the printedpattern on a bill is encountered relative to the reflectance valueobtained at the edge of the bill where no printed pattern exist.According to a preferred embodiment of this invention, illuminatedstrips of at least two different dimensions are used for the scanningprocess. A narrow strip is used initially to detect the starting pointof the printed pattern on a bill and is adapted to distinguish the thinborderline that typically marks the starting point of and encloses theprinted pattern on a bill. For the rest of the narrow dimension scanningfollowing detection of the border line of the printed pattern, asubstantially wider strip of light is used to collect the predefinednumber of samples for a bill scan the generation and storage ofcharacteristic patterns using standard notes and the subsequentcomparison and correlation procedure for classifying the scanned bill asbelonging to one of several predefined currency denominations is basedon the above-described sensing and correlation technique.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description in conjunction with thedrawings in which:

FIG. 1 is a functional block diagram illustrating the conceptual basisfor the optical sensing and correlation method and apparatus, accordingto the system of this invention;

FIG. 1A is a diagrammatic perspective illustration of the successiveareas scanned during the traversing movement of a single bill across thescanhead;

FIG. 1B is a perspective view of a bill and the preferred area to bescanned on the bill;

FIG. 1C is a diagrammatic side elevation of the scan areas illustratedin FIG. 1A, to show the overlapping relationship of those areas;

FIGS. 2-2A are block diagrams illustrating a preferred circuitarrangement for processing and correlating reflectance data according tothe optical sensing and counting technique of this invention;

FIGS. 3-8A are flow charts illustrating the sequence of operationsinvolved in implementing the optical sensing and correlation technique;

FIGS. 9A-C are graphical illustrations of representative characteristicpatterns generated by narrow dimension optical scanning of a currencybill;

FIGS. 10A-E are graphical illustrations of the effect produced oncorrelation pattern by using the progressive shifting technique,according to an embodiment of this invention;

FIG. 11 is a perspective view showing currency discrimination andcounting apparatus particularly adapted to and embodying the opticalsensing and correlation technique of this invention;

FIG. 12 is a partial perspective view illustrating the mechanism usedfor separating currency bills and injecting them in a sequential fashioninto the transport path;

FIG. 13 is a side view of the apparatus of FIG. 11 illustrating theseparation mechanism and the transport path;

FIG. 14 is a side view of the apparatus of FIG. 11 illustrating detailsof the drive mechanism;

FIG. 15 is a top view of the currency discriminating and countingapparatus shown in FIGS. 11-14;

FIG. 16 is an exploded top perspective view of the optical scanhead usedin the system of FIGS. 1-15;

FIG. 17 is a bottom perspective view of the scanhead of FIG. 16, withthe body portion of the scanhead sectioned along a vertical planepassing through the wide slit at the top of the scanhead;

FIG. 18 is a bottom perspective view of the scanhead of FIG. 16, withthe body portion of the scanhead sectioned along a vertical planepassing through the narrow slit at the top of the scanhead;

FIG. 19 is an illustration of the light distribution produced about theoptical scanhead; and

FIG. 20 is a diagrammatic illustration of the location of two auxiliaryphoto sensors relative to a bill passed thereover by the transportmechanism shown in FIGS. 11-15; and

FIGS. 21-24 are flow charts illustrating the sequence of operationsinvolved in various enhancements to the operating program for the basicoptical sensing and correlation process.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and will herein be described in detail. Itshould be understood, however, that it is not intended to limit theinvention to the particular forms disclosed, but on the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theappended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is shown a functional block diagramillustrating the optical sensing and correlation system according tothis invention. The system 10 includes a bill accepting station 12 wherestacks of currency bills that need to be identified and counted arepositioned. Accepted bills are acted upon by a bill separating station14 which functions to pick out or separate one bill at a time for beingsequentially relayed by a bill transport mechanism 16, according to aprecisely predetermined transport path, across an optical scanhead 18where the currency denomination of the bill is scanned, identified andcounted at a rate in excess of 800 bills per minute. The scanned bill isthen transported to a bill stacking station 20 where bills so processedare stacked for subsequent removal.

The optical scanhead 18 comprises at least one light source 22 directinga beam of coherent light downwardly onto the bill transport path so asto illuminate a substantially rectangular light strip 24 upon a currencybill 17 positioned on the transport path below the scanhead 18. Lightreflected off the illuminated strip 24 is sensed by a photodetector 26positioned directly below the strip. The analog output of photodetector26 is converted into a digital signal by means of an analog-to-digital(ADC) convertor unit 28 whose output is fed as a digital input to acentral processing unit (CPU) 30.

According to a feature of this invention, the bill transport path isdefined in such a way that the transport mechanism 16 moves currencybills with the narrow dimension "W" of the bills being parallel to thetransport path and the scan direction. Thus, as a bill 17 moves on thetransport path on the scanhead 18, the coherent light strip 24effectively scans the bill across the narrow dimension "W" of the bill.Preferably, the transport path is so arranged that a currency bill 17 isscanned approximately about the central section of the bill along itsnarrow dimension, as best shown in FIG. 1. The scanhead 18 functions todetect light reflected from the bill as it moves across the illuminatedlight strip 24 and to provide an analog representation of the variationin light so reflected which, in turn, represents the variation in thedark and light content of the printed pattern or indicia on the surfaceof the bill. This variation in light reflected from the narrow dimensionscanning of the bills serves as a measure for distinguishing, with ahigh degree of confidence, among a plurality of currency denominationswhich the system of this invention is programmed to handle.

A series of such detected reflectance signals are obtained across thenarrow dimension of the bill, or across a selected segment thereof, andthe resulting analog signals are digitized under control of the CPU 30to yield a fixed number of digital reflectance data samples. The datasamples are then subjected to a digitizing process which includes anormalizing routine for processing the sampled data for improvedcorrelation and for smoothing out variations due to "contrast"fluctuations in the printed pattern existing on the bill surface. Thenormalized reflectance data so digitized represents a characteristicpattern that is fairly unique for a given bill denomination and providessufficient distinguishing features between characteristic patterns fordifferent currency denominations, as will be explained in detail below.

In order to ensure strict correspondence between reflectance samplesobtained by narrow dimension scanning of successive bills, theinitiation of the reflectance sampling process is preferably controlledthrough the CPU 30 by means of an optical encoder 32 which is linked tothe bill transport mechanism 16 and precisely tracks the physicalmovement of the bill 17 across the scanhead 18. More specifically, theoptical encoder 32 is linked to the rotary motion of the drive motorwhich generates the movement imparted to the bill as it is relayed alongthe transport path. In addition, it is ensured that positive contact ismaintained between the bill and the transport path, particularly whenthe bill is being scanned by the scanhead 18. Under these conditions,the optical encoder is capable of precisely tracking the movement of thebill relative to the light strip generated by the scanhead by monitoringthe rotary motion of the drive motor.

The output of photodetector 26 is monitored by the CPU 30 to initiallydetect the presence of the bill underneath the scanhead and,subsequently, to detect the starting point of the printed pattern on thebill, as represented by the thin borderline 17B which typically enclosesthe printed indicia on currency bills. Once the borderline 17B has beendetected, the optical encoder is used to control the timing and numberof reflectance samples that are obtained from the output of thephotodetector 26 as the bill 17 moves across the scanhead 18 and isscanned along its narrow dimension.

The detection of the borderline constitutes an important step andrealizes improved discrimination efficiency since the borderline servesas an absolute reference point for initiation of sampling. If the edgeof a bill were to be used as a reference point, relative displacement ofsampling points can occur because of the random manner in which thedistance from the edge to the borderline varies from bill to bill due tothe relatively large range of tolerances permitted during printing andcutting of currency bills. As a result, it becomes difficult toestablish direct correspondence between sample points in successive billscans and the discrimination efficiency is adversely affected.

The use of the optical encoder for controlling the sampling processrelative to the physical movement of a bill across the scanhead is alsoadvantageous in that the encoder can be used to provide a predetermineddelay following detection of the borderline prior to initiation ofsamples. The encoder delay can be adjusted in such a way that the billis scanned only across those segments along its narrow dimension whichcontain the most distinguishable printed indicia relative to thedifferent currency denominations.

In the case of U.S. currency, for instance, it has been determined thatthe central, approximately two-inch portion of currency bills, asscanned across the central section of the narrow dimension of the bill,provides sufficient data for distinguishing among the various U.S.currency denominations on the basis of the correlation technique used inthis invention. Accordingly, the optical encoder can be used to controlthe scanning process so that reflectance samples are taken for a setperiod of time and only after a certain period of time has elapsed sincethe borderline has been detected, thereby restricting the scanning tothe desired central portion of the narrow dimension of the bill.

FIGS. 1A-1C illustrate the scanning process in more detail. As a bill isadvanced in a direction parallel to the narrow edges of the bill,scanning via the wide slit in the scanhead is effected along a segment Sof the central portion of the bill. This segment S begins a fixeddistance d inboard of the border line B. As the bill traverses the scanhead, a strip s of the segment S is always illuminated, and thephotodector produces a continuous output signal which is proportional tothe intensity of the light reflected from the illuminated strip s at anygiven instant. This output is sampled at intervals controlled by theencoder, so that the sampling intervals are precisely synchronized withthe movement of the bill across the scanhead.

As illustrated in FIGS. 1A and 1C, it is preferred that the samplingintervals be selected so that the strips s that are illuminated forsuccessive samples overlap one another. The odd-numbered andeven-numbered sample strips have been separated in FIGS. 1A and 1C tomore clearly illustrate this overlap. For example, the first and secondstrips s1 and s2 overlap each other, the second and third strips s2 ands3 overlap each other, and so on. Each adjacent pair of strips overlapeach other. In the illustrative example, this is accomplished bysampling strips that are 0.050 inch wide at 0.029 inch intervals, alonga segment S that is 1.83 inch long (64 samples).

The optical sensing and correlation technique is based upon using theabove process to generate a series of master characteristic patternsusing standard bills for each denomination of currency that is to bedetected. According to a preferred embodiment, two or fourcharacteristic patterns are generated and stored within system memory,preferably in the form of an EPROM 34 (see FIG. 1), for each detectablecurrency denomination. The characteristic patterns for each bill aregenerated from optical scans, performed on the green surface of the billand taken along both the "forward" and "reverse" directions relative tothe pattern printed on the bill.

In adapting this technique to U.S. currency, for example, characteristicpatterns are generated and stored for seven different denominations ofU.S. currency, i.e., $1, $2, $5, $10, $20, $50, and $100. As explainedpreviously, four characteristic patterns are generated for the $10 bill,and two characteristic patterns are generated for each of the otherdenominations. Accordingly, a master set of 16 different characteristicpatterns is stored within the system memory for subsequent correlationpurposes. Once the master characteristic patterns have been stored, thepattern generated by scanning a bill under test is compared by the CPU30 with each of the 16 pre-stored master characteristic patterns togenerate, for each comparison, a correlation number representing theextent of correlation, i.e., similarity between corresponding ones ofthe plurality of data samples, for the patterns being compared.

The CPU 30 is programmed to identify the denomination of the scannedbill as corresponding to the stored characteristic pattern for which thecorrelation number resulting from pattern comparison is found to be thehighest. In order to preclude the possibility of mischaracterizing thedenomination of a scanned bill, as well as to reduce the possibility ofspurious notes being identified as belonging to a valid denomination, abi-level threshold of correlation is used as the basis for making a"positive" call, as will be explained in detail below.

Using the above sensing and correlation approach, the CPU 30 isprogrammed to count the number of bills belonging to a particularcurrency denomination as part of a given set of bills that have beenscanned for a given scan batch, and to determine the aggregate total ofthe currency amount represented by the bills scanned during a scanbatch. The CPU 30 is also linked to an output unit 36 which is adaptedto provide a display of the number of bills counted, the breakdown ofthe bills in terms of currency denomination, and the aggregate total ofthe currency value represented by counted bills. The output unit 36 canalso be adapted to provide a print-out of the displayed information in adesired format.

Referring now to FIG. 2, there is shown a representation, in blockdiagram form, of a preferred circuit arrangement for processing andcorrelating reflectance data according to the system of this invention.As shown therein, the CPU 30 accepts and processes a variety of inputsignals including those from the optical encoder 32, the photodetector26 and a memory unit 38, which can be an erasable programmable read onlymemory (EPROM). The memory unit 38 has stored within it the correlationprogram on the basis of which patterns are generated and test patternscompared with stored master programs in order to identify thedenomination of test currency. A crystal 40 serves as the time base forthe CPU 30, which is also provided with an external reference voltageV_(REF) on the basis of which peak detection of sensed reflectance datais performed, as explained in detail below.

The CPU 30 also accepts a timer reset signal from a reset unit 44 which,as shown in FIG. 2A, accepts the output voltage from the photodetector26 and compares it, by means of a threshold detector 44A, relative to apre-set voltage threshold, typically 5.0 volts, to provide a resetsignal which goes "high" when a reflectance value corresponding to thepresence of paper is sensed. More specifically, reflectance sampling isbased on the premise that no portion of the illuminated light strip (24in FIG. 1) is reflected to the photodetector in the absence of a billpositioned below the scanhead. Under these conditions, the output of thephotodetector represents a "dark" or "zero" level reading. Thephotodetector output changes to a "white" reading, typically set to havea value of about 5.0 volts, when the edge of a bill first becomespositioned below the scanhead and falls under the light strip 24. Whenthis occurs, the reset unit 44 provides a "high" signal to the CPU 30and marks the initiation of the scanning procedure.

In accordance with a feature of this invention, the machine-directiondimension of the illuminated strip of light produced by the lightsources within the scanhead is set to be relatively small for theinitial stage of the scan when the thin borderline is being detected.The use of the narrow slit increases the sensitivity with which thereflected light is detected and allows minute variations in the "gray"level reflected off the bill surface to be sensed. This is important inensuring that the thin borderline of the pattern, i.e., the startingpoint of the printed pattern on the bill, is accurately detected. Oncethe borderline has been detected, subsequent reflectance sampling isperformed on the basis of a relatively wider light strip in order tocompletely scan across the narrow dimension of the bill and obtain thedesired number of samples, at a rapid rate. The use of a wider slit forthe actual sampling also smooths out the output characteristics of thephotodetector and realizes the relatively large magnitude of analogvoltage which is essential for accurate representation and processing ofthe detected reflectance values.

Returning to FIG. 2, the CPU 30 processes the output of photodetector 26through a peak detector 50 which essentially functions to sample thephotodetector output voltage and hold the highest, i.e., peak, voltagevalue encountered after the detector has been enabled. The peak detectoris also adapted to define a scaled voltage on the basis of which thepattern borderline on bills is detected. The output of the peak detector50 is fed to a voltage divider 54 which lowers the peak voltage down toa scaled voltage V_(S) representing a predefined percentage of this peakvalue. The voltage V_(S) is based upon the percentage drop in outputvoltage of the peak detector as it reflects the transition from the"high" reflectance value resulting from the scanning of the unprintededge portions of a currency bill to the relatively lower "gray"reflectance value resulting when the thin borderline is encountered.Preferably, the scaled voltage V_(S) is set to be about 70-80 percent ofthe peak voltage.

The scaled voltage V_(S) is supplied to a line detector 56 which is alsoprovided with the incoming instantaneous output of the photodetector 26.The line detector 56 compares the two voltages at its input side andgenerates a signal L_(DET) which normally stays "low" and goes "high"when the edge of the bill is scanned. The signal L_(DET) goes "low" whenthe incoming photodetector output reaches the pre-defined percentage ofthe peak photodetector output up to that point, as represented by thevoltage V_(S). Thus, when the signal L_(DET) goes "low", it is anindication that the borderline of the bill pattern has been detected. Atthis point, the CPU 30 initiates the actual reflectance sampling undercontrol of the encoder 32 (see FIG. 2) and the desired fixed number ofreflectance samples are obtained as the currency bill moves across theilluminated light strip and is scanned along the central section of itsnarrow dimension.

When master characteristic patterns are being generated, the reflectancesamples resulting from the scanning of a standard bill are loaded intocorresponding designated sections within a system memory 60, which ispreferably an EPROM. The loading of samples is accomplished through abuffered address latch 58, if necessary. Preferably, master patterns aregenerated by scanning a standard bill a plurality of times, typicallythree (3) times, and obtaining the average of corresponding data samplesbefore storing the average as representing a master pattern. Duringcurrency discrimination, the reflectance values resulting from thescanning of a test bill are sequentially compared, under control of thecorrelation program stored within the memory unit 38, with each of thecorresponding characteristic patterns stored within the EPROM 60, againthrough the address latch 58.

Referring now to FIGS. 3-7, there are shown flow charts illustrating thesequence of operations involved in implementing the above-describedoptical sensing and correlation technique of this invention. FIG. 3, inparticular, illustrates the sequence involved in detecting the presenceof a bill under the scanhead and the borderline on the bill. Thissection of the system program, designated as "TRIGGER", is initiated atstep 70. At step 71 a determination is made as to whether or not astart-of-note interrupt, which signifies that the system is ready tosearch for the presence of a bill, is set, i.e., has occurred. If theanswer at step 71 is found to be positive, step 72 is reached where thepresence of the bill adjacent the scanhead is ascertained on the basisof the reset procedure described above in connection with the reset unit44 of FIG. 2.

If the answer at step 72 is found to be positive, i.e., a bill is foundto be present, step 73 is reached where a test is performed to see ifthe borderline has been detected on the basis of the reduction in peakvalue to a predefined percentage thereof, which, as described above, isindicated by the signal L_(DET) going "low." If the answer at step 73 isfound to be negative, the program continues to loop until the borderlinehas been detected. If the answer at step 72 is found to be negative,i.e., no bill is found to be present, the start-of-note interrupt isreset at step 74 and the program returns from interrupt at step 75.

If the borderline is found to have been detected at step 73, step 76 isaccessed where an A/D completion interrupt is enabled, therebysignifying that the analog-to-digital conversion can subsequently beperformed at desired time intervals. Next, at step 77, the time when thefirst reflectance sample is to be obtained is defined, in conjunctionwith the output of the optical encoder. At step 78 the capture anddigitization of the detected reflectance samples is undertaken byrecalling a routine designated as "STARTA2D" which will be described indetail below. At the completion of the digitization process, anend-of-note interrupt must occur, which resets the system for sensingthe presence of the following bill to be scanned, which is enabled atstep 79. Subsequently, at step 80 the program returns from interrupt.

If the start-of-note interrupt is not found to have occurred at step 71,a determination is made at step 81 to see if the end-of-note interrupthas occurred. If the answer at 81 is negative, the program returns frominterrupt at step 85. If a positive answer is obtained at 81, step 83 isaccessed where the start-of-note interrupt is activated and, at step 84,the reset unit, which monitors the presence of a bill, is reset to beready for determining the presence of bills. Subsequently, the programreturns from interrupt at step 85.

Referring now to FIGS. 4A and 4B there are shown, respectively, routinesfor starting the STARTA2D routine and the digitizating routine itself.In FIG. 4A, the initiation of the STARTA2D routine at step 90 causes thesample pointer, which provides an indication of the sample beingobtained and digitized at a given time, to be initialized. Subsequently,at step 91, the particular channel on which the analog-to-digitalconversion is to be performed is enabled. The interrupt authorizing thedigitization of the first sample is enabled at step 92 and the mainprogram accessed again at step 93.

FIG. 4B is a flow chart illustrating the sequential procedure involvedin the analog-to-digital conversion routine, which is designated as"A2D". The routine is started at step 100. Next, the sample pointer isdecremented at step 101 so as to maintain an indication of the number ofsamples remaining to be obtained. At step 102, the digital datacorresponding to the output of the photodetector for the current sampleis read. The data is converted to its final form at step 103 and storedwithin a pre-defined memory segment as X_(IN).

Next, at step 105, a check is made to see if the desired fixed number ofsamples "N" has been taken. If the answer is found to be negative, step106 is accessed where the interrupt authorizing the digitization of thesucceeding sample is enabled and the program returns from interrupt atstep 107 for completing the rest of the digitizing process. However, ifthe answer at step 105 is found to be positive, i.e., the desired numberof samples have already been obtained, a flag indicating the same is setat step 108 and the program returns from interrupt at step 109.

Referring now to FIG. 5, there is shown the sequential procedureinvolved in executing the routine, designated as "EXEC", which performsthe mathematical steps involved in the correlation process. The routineis started at step 110. At step 111, all interrupts are disabled whileCPU initialization occurs. At step 112, any constants associated withthe sampling process are set and, at step 113, communications protocols,if any, for exchange of processed data and associated results, badrates, interrupt masks, etc. are defined.

At step 114, the reset unit indicating the presence of a bill is resetfor detecting the presence of the first bill to be scanned. At step 115,the start-of-note interrupt is enabled to put the system on the look outfor the first incoming bill. Subsequently, at step 116, all otherrelated interrupts are also enabled since, at this point, theinitialization process has been completed and the system is ready tobegin scanning bills. A check is made at step 117 to see if, in fact,all the desired number of samples have been obtained. If the answer atstep 117 is found to be negative the program loops until a positiveanswer is obtained.

In accordance with this invention, a simple correlation procedure isutilized for processing digitized reflectance values into a form whichis conveniently and accurately compared to corresponding valuespre-stored in an identical format. More specifically, as a first step,the mean value X for the set of digitized reflectance samples (comparing"n" samples) obtained for a bill scan run is first obtained as below:##EQU1##

Subsequently, a normalizing factor Sigma "σ" is determined as beingequivalent to the sum of the square of the difference between eachsample and the mean, as normalized by the total number n of samples.More specifically, the normalizing factor is calculated as below:##EQU2##

In the final step, each reflectance sample is normalized by obtainingthe difference between the sample and the above-calculated mean valueand dividing it by the square root of the normalizing factor Sigma "σ"as defined by the following equation: ##EQU3##

The result of using the above correlation equations is that, subsequentto the normalizing process, a relationship of correlation exists betweena test pattern and a master pattern such that the aggregate sum of theproducts of corresponding samples in a test pattern and any masterpattern, when divided by the total number of samples, equals unity ifthe patterns are identical. Otherwise, a value less than unity isobtained. Accordingly, the correlation number or factor resulting fromthe comparison of normalized samples within a test pattern to those of astored master pattern provides a clear indication of the degree ofsimilarity or correlation between the two patterns.

According to a preferred embodiment of this invention, the fixed numberof reflectance samples which are digitzed and normalized for a bill scanis selected to be 64. It has experimentally been found that the use ofhigher binary orders of samples (such as 128, 256, etc.) does notprovide a correspondingly increased discrimination efficiency relativeto the increased processing time involved in implementing theabove-described correlation procedure. It has also been found that theuse of a binary order of samples lower than 64, such as 32, produces asubstantial drop in discrimination efficiency.

The correlation factor can be represented conveniently in binary termsfor ease of correlation. In a preferred embodiment, for instance, thefactor of unity which results when a hundred percent correlation existsis represented in terms of the binary number 2¹⁰, which is equal to adecimal value of 1024. Using the above procedure, the normalized sampleswithin a test pattern are compared to each of the 16 mastercharacteristic patterns stored within the system memory in order todetermine the particular stored pattern to which the test patterncorresponds most closely by identifying the comparison which yields acorrelation number closest to 1024.

According to a feature of this invention, a bi-level threshold ofcorrelation is required to be satisfied before a particular call ismade, for at least certain denominations of bills. More specifically,the correlation procedure is adapted to identify the two highestcorrelation numbers resulting from the comparison of the test pattern toone of the stored patterns. At that point, a minimum threshold ofcorrelation is required to be satified by these two correlation numbers.It has experimentally been found that a correlation number of about 850serves a as a good cut-off threshold above which positive calls may bemade with a high degree of confidence and below which the designation ofa test pattern as corresponding to any of the stored patterns isuncertain. As a second thresholding level, a minimum separation isprescribed between the two highest correlation numbers before making acall. This ensures that a positve call is made only when a test patterndoes not correspond, within a given range of correlation, to more thanone stored master pattern. Preferably, the minimum separation betweencorrelation numbers is set to be 150 when the highest correlation numberis between 800 and 850. When the highest correlation number is below800, no call is made.

Returning now to FIG. 5, the correlation procedure is initiated at step119 where a routine designated as "PROCESS" is accessed. The procedureinvolved in executing this routine is illustrated at FIG. 6A which showsthe routine starting at step 130. At step 131, the mean X is calculatedin accordance on the basis of Equation (1). At step 132 the sum of thesquares is calculated in accordance with Equation (2). At step 133, thedigitized values of the reflectance samples, as represented in integerformat XIN, are converted to floating point format XFLOAT for furtherprocessing. At step 134, a check is made to see if all samples have beenprocessed and if the answer is found to be positive, the routine ends atstep 135 and the main program is accessed again. If the answer at step134 is found to be negative, the routine returns to step 132 where theabove calculations are repeated.

At the end of the routine PROCESS, the program returns to the routineEXEC at step 120 where the flag indicating that all digitizedreflectance samples have been processed is reset. Subsequently, at step121, a routine designated as "SIGCAL" is accessed. The procedureinvolved in executing this routine is illustrated at FIG. 6B which showsthe routine starting at step 140. At step 141, the square root of thesum of the squares, as calculated by the routine PROCESS, is calculatedin accordance with Equation (2). At step 142, the floating point valuescalculated by the routine PROCESS are normalized in accordance withEquation (3) using the calculated values at step 141. At step 143, acheck is made to see if all digital samples have been processed. If theanswer at step 143 is found to be negative, the program returns to step142 and the conversion is continued until all samples have beenprocessed. At that point, the answer at step 143 is positive and theroutine returns to the main program at step 144.

Returning to the flow chart of FIG. 5, the next step to be executed isstep 122 where a routine designated as "CORREL" is accessed. Theprocedure involved in executing this routine is illustrated at FIG. 7which shows the routine starting at 150. At step 151, correlationresults are initialized to zero and, at step 152, the test pattern iscompared to the first one of the stored master patterns. At step 153,the first call corresponding to the highest correlation number obtainedup to that point is determined. At step 154, the second callcorresponding to the second highest correlation number obtained up tothat point is determined. At step 155, a check is made to see if thetest pattern has been compared to all master patterns. If the answer isfound to be negative, the routine reverts to step 152 where thecomparison procedure is reiterated. When all master patterns have beencompared to the test pattern, step 155 yields a positive result and theroutine returns to the main program at step 156.

Returning again to FIG. 5, step 124 is accessed where a routinedesignated as "SEROUT" is initiated. The procedure involved in executingthe routine SEROUT is illustrated at FIG. 8 which shows the routine asstarting at step 160. Step 161 determines whether the correlation numberis greater than 799. If the answer is negative, the correlation numberis too low to identify the denomination of the bill with certainty, andthus step 162 generates a "no call" code and returns to the main programat step 163.

An affirmative answer at step 161 advances the system to step 164, whichdetermines whether the correlation number is greater than 849. Anaffirmative answer at step 164 indicates that the correlation number issufficiently high that the denomination of the scanned bill can beidentified with certainty without any further checking. Consequently, a"denomination" code identifying the denomination represented by thestored pattern resulting in the highest correlation number is generatedat step 165, and the system returns to the main program at step 163.

A negative answer at step 164 indicates that the correlation number isbetween 800 and 850. It has been found that correlation numbers withinthis range are sufficient to identify $1 and $5 bills, but not otherdenominations of bills. Accordingly, a negative response at step 164advances the system to step 166 which determines whether the differencebetween the two highest correlation numbers is greater than 149. If theanswer is affirmative, the denomination identified by the highestcorrelation number is acceptable, and thus the "denomination" code isgenerated at step 165.

If the difference between the two highest correlation numbers is lessthan 150, step 166 produces a negative response which advances thesystem to step 167 to determine whether the highest correlation numberidentified the bill as either a $1-bill or a $5-bill. If the answer isaffirmative, the highest correlation number is acceptable as identifyingthe bill denomination, and thus the "denomination" code is generated atstep 165. A negative response at step 167 indicates that the bill wasnot identified as a $1-bill or a $5-bill by the highest correlationnumber, the difference between the two highest correlation numbers wasless than 150, and the highest correlation number was less then 850.This combination of conditions indicates that a positive call cannot bemade with a high degree of confidence, and thus the "no call" code isgenerated at step 162.

One problem encountered in currency recognition and counting systems ofthe above-described kind is the difficulty involved in interrupting (fora variety of reasons) and resuming the scanning and counting procedureas a stack of bills is being scanned. If a particular currencyrecognition unit (CRU) has to be halted in operation due to a "major"system error, such as a bill being jammed along the transport path,there is generally no concern about the outstanding transitional statusof the overall recognition and counting process. However, where the CRUhas to be halted due to a "minor" error, such as the identification of ascanned bill as being a counterfeit (based on a variety of monitoredparameters which are not pertinent to the present disclosure) or a "nocall" (a bill which is not identifiable as belonging to a specificcurrency denomination based on the plurality of stored master patternsand/or other criteria), it is desirable that the transitional status ofthe overall recognition and counting process be retained so that the CRUmay be restarted without any effective disruptions of therecognition/counting process.

More specifically, once a scanned bill has been identified as a "nocall" bill (B₁) based on some set of predefined criteria, it isdesirable that this bill B₁ be transported directly to the systemstacker and the CRU brought to a halt with bill B₁ remaining at thetop-most stacker position while, at the same time, ensuring that thefollowing bills are maintained in positions along the bill transportpath whereby CRU operation can be conveniently resumed without anydisruption of the recognition/counting process.

Since the bill processing speeds at which currency recognition systemsmust operate are substantially high (speeds of the order of about 1000bills per minute are desirable), it is practically impossible to totallyhalt the system following a "no call" without the following bill B₂already being transported under the optical scanhead and partiallyscanned. As a result, it is virtually impossible for the CRU system toretain the transitional status of the recognition/counting process(particularly with respect to bill B₂) in order that the process may beresumed once the bad bill B₁ has been transported to the stacker,conveniently removed therefrom, and the system restarted. The basicproblem is that if the CRU is halted with bill B₂ only partiallyscanned, there is no possibility of referencing the data reflectancesamples extracted therefrom in such a way that the scanning may be latercontinued (when the CRU is restarted) from exactly the same point wherethe sample extraction process was interrupted when the CRU was stopped.

Even if an attempt were made at immediately halting the CRU systemfollowing a "no call," any subsequent scanning of bills would be totallyunreliable because of mechanical backlash effects and the resultantdisruption of the optical encoder routine used for bill scanning.Consequently, when the CRU is restarted, the call for the following billis also likely to be bad and the overall recognition/counting process istotally disrupted as a result of an endless loop of "no calls."

According to an important feature of the present invention, the aboveproblems are solved by an improved currency detecting and countingtechnique whereby a scanned bill identified as a "no call" istransported directly to the top of the system stacker and the CRU ishalted without adversely affecting the data collection and processingsteps for a succeeding bill. Accordingly, when the CRU is restarted, theoverall bill recognition and counting procedure can be resumed withoutany disruption as if the CRU had never been halted at all.

According to the improved currency detecting/counting technique, the CRUis operated in the normal fashion described above in detail, whereby anincoming bill is scanned and processed in order to make a call as to thebill denomination. If the bill is identified as a "no call" based on anyof a variety of conventionally defined bill criteria (such as thecriteria in FIG. 8), the CRU is subjected to a controlled decelerationprocess whereby the CRU operating speed, i.e., the speed at which testbills are moved across the system scanhead along the bill transportpath, is reduced from its normal operating level. During thisdeceleration process the "no call" bill (B₁) is transported to the topof the stacker and, at the same time, the following bill B₂ is subjectedto the standard scan and processing procedure in order to identify thedenomination thereof.

The rate of deceleration is such that optical scanning of bill B₂ iscompleted by the time the CRU operating speed is reduced to a predefinedoperating speed. While the exact operating speed at the end of thescanning of bill B₂ is not critical, the objective is to permit completescanning of bill B₂ without subjecting it to backlash effects that wouldresult if the ramping were too fast while, at the same time, ensuringthat the bill B₁ has in fact been transported to the stacker in themeantime.

It has experimentally been determined that at nominal operating speedsof the order of 1000 bills per minute, the deceleration is preferablysuch that the CRU operating speed is reduced to about one-third of itsnormal operating speed at the end of the deceleration phase, i.e., bythe time optical scanning of bill B₂ has been completed. It has beendetermined that at these speed levels, positive calls can be made as tothe denomination of bill B₂ based on reflectance samples gathered duringthe deceleration phase with a relatively high degree of certainty (i.e.,with a correlation number exceeding about 850.)

Once the optical scanning of bill B₂ has been completed, the speed isreduced to an even slower speed until the bill B₂ has passed bill-edgesensors S1 and S2 described below whereby it is then brought to acomplete stop. At the same time, the results of the processing ofscanned data corresponding to bill B₂ are stored in system memory. Theultimate result of this stopping procedure is that the CRU is brought toa complete halt following the point where the scanning of bill B₂ hasbeen reliably completed since the scan procedure is not subjected to thedisruptive effects (backlash, etc.) which would result if a completehalt were attempted immediately after bill B₁ is identified as a "nocall."

More importantly, the reduced operating speed of the machine at the endof the deceleration phase is such that the CRU can be brought to a totalhalt before the next following bill B₃ has been transported over theoptical scanhead. Thus, when the CRU is in fact halted, bill B₁ ispositioned at the top of the system stacker, bill B₂ is maintained intransit between the optical scanhead and the stacker after it has beensubjected to scanning, and the following bill B₃ is stopped short of theoptical scanhead.

When the CRU is restarted, presumably after corrective action has beentaken responsive to the "minor" error which led to the CRU being stopped(such as the removal of the "no call" bill from the top of the stacker),the overall bill recognition/counting operation can be resumed in anuninterrupted fashion by using the stored call results for bill B₂ asthe basis for updating the system count appropriately, moving bill B₂from its earlier transitional position along the transport path into thestacker, and moving bill B₃ along the transport path into the opticalscanhead area where it can be subjected to normal scanning andprocessing. A routine for executing the deceleration/stopping proceduredescribed above is illustrated by the flow chart in FIG. 8A. Thisroutine is initiated at step 170 with the CRU in its normal operatingmode. At step 171, a test bill B₁ is scanned and the data reflectancesamples resulting therefrom are processed. Next, at step 172, adetermination is made as to whether or not test bill B₁ is a "no call"using predefined criteria in combination with the overall billrecognition procedure, such as the routine of FIG. 8. If the answer atstep 172 is negative, i.e., the test bill B₁ can be identified, step 173is accessed where normal bill processing is continued in accordance withthe procedures described above. If, however, the test bill B₁ is foundto be bad at step 172, step 174 is accessed where CRU slowdown isinitiated, e.g., the transport drive motor speed is reduced to aboutone-third its normal speed.

Subsequently, the bad bill B₁ is guided to the stacker while, at thesame time, the following test bill B₂ is brought under the opticalscanhead and subjected to the scanning and processing steps. The callresulting from the scanning and processing of bill B₂ is stored insystem memory at this point. Step 175 determines whether the scanning ofbill B₂ is complete. When the answer is negative, step 176 determineswhether a preselected "bill timeout" period has expired so that thesystem does not wait for the scanning of a bill that is not present. Anaffirmative answer at step 176 returns the system to the main program atstep 175 while a negative answer at step 176 causes steps 175 and 176 tobe reiterated until one of them produces an affirmative response.

An affirmative response at step 175 causes step 177 to further reducethe speed of the transport drive motor, i.e., to one-sixth the normalspeed. Before stopping the transport drive motor, step 178 determineswhether either of the sensors S1 or S2 (described below) is covered by abill. A negative answer at step 178 indicates that the bill has clearedboth sensors S1 and S2, and thus the transport drive motor is stopped atstep 179. This signifies the end of the deceleration/stopping process.At this point in time, bill B₂ remains in transit while the followingbill B₃ is stopped on the transport path just short of the opticalscanhead.

Following step 179, corrective action responsive to the identificationof a "no call" bill is conveniently undertaken; the top-most bill in thestacker is easily removed therefrom and the CRU is then in condition forresuming the recognition/counting process. Accordingly, the CRU can berestarted and the stored results corresponding to bill B₂, are used toappropriately update the system count. Next, the identified bill B₂ isguided along the transport path to the stacker, and the CRU continueswith its normal processing routine.

Referring now to FIGS. 9A-C there are shown three test patternsgenerated, respectively, for the forward scanning of a $1 bill along itsgreen side, the reverse scanning of a $2 bill on its green side, and theforward scanning of a $100 bill on its green side. It should be notedthat, for purposes of clarity the test patterns in FIGS. 9A-C weregenerated by using 128 reflectance samples per bill scan, as opposed tothe preferred use of only 64 samples. The marked difference existingbetween corresponding samples for these three test patterns isindicative of the high degree of confidence with which currencydenominations may be called using the foregoing optical sensing andcorrelation procedure.

The optical sensing and correlation technique described above permitsidentification of pre-programmed currency denominations with a highdegree of accuracy and is based upon a relatively low processing timefor digitizing sampled reflectance values and comparing them to themaster characteristic patterns. The approach is used to scan currencybills, normalize the scanned data and generate master patterns in such away that bill scans during operation have a direct correspondencebetween compared sample points in portions of the bills which possessthe most distinguishable printed indicia. A relatively low number ofreflectance samples is required in order to be able to adequatelydistinguish between several currency denominations.

A major advantage with this approach is that it is not required thatcurrency bills be scanned along their wide dimensions. Further, thereduction in the number of samples reduces the processing time to suchan extent that additional comparisons can be made during the timeavailable between the scanning of successive bills. More specifically,as described above, it becomes possible to compare a test pattern withtwo or more stored master characteristic patterns so that the system ismade capable of identifying currency which is scanned in the "forward"or "reverse" directions along the green surface of the bill.

Another advantage accruing from the reduction in processing timerealized by the present sensing and correlation scheme is that theresponse time involved in either stopping the transport of a bill thathas been identified as "spurious", i.e., not corresponding to any of thestored master characteristic patterns, or diverting such a bill to aseparate stacker bin, is correspondingly shortened. Accordingly, thesystem can conveniently be programmed to set a flag when a scannedpattern does not correspond to any of the master patterns. Theidentification of such a condition can be used to stop the billtransport drive motor for the mechanism. Since the optical encoder istied to the rotational movement of the drive motor, synchronism can bemaintained between pre- and post-stop conditions. In the dual-processorimplementation discussed above, the information concerning theidentification of a "spurious" bill would be included in the informationrelayed to the general processor unit which, in turn, would control thedrive motor appropriately.

The correlation procedure and the accuracy with which a denomination isidentified directly relates to the degree of correspondence betweenreflectance samples on the test pattern and corresponding samples on thestored master patterns. Thus, shrinkage of "used" bills which, in turn,causes corresponding reductions in their narrow dimensions, can possiblyproduce a drop in the degree of correlation between such used bills of agiven denomination and the corresponding master patterns. Currency billswhich have experienced a high degree of usage exhibit such a reductionin both the narrow and wide dimensions of the bills. While the sensingand correlation technique of this invention remains relativelyindependent of any changes in the wide dimension of bills, reductionalong the narrow dimension can affect correlation factors by realizing arelative displacement of reflectance samples obtained as the "shrunk"bills are transported across the scanhead.

In order to accommodate or nullify the effect of such narrow dimensionshrinking, the above-described correlation technique can be modified byuse of a progressive shifting approach whereby a test pattern which doesnot correspond to any of the master patterns is partitioned intopredefined sections, and samples in successive sections areprogressively shifted and compared again to the stored patterns in orderto identify the denomination. It has experimentally been determined thatsuch progressive shifting effectively counteracts any sampledisplacement resulting from shrinkage of a bill along its narrowdimension.

The progressive shifting effect is best illustrated by the correlationpatterns shown in FIGS. 10A-D. For purposes of clarity, the illustratedpatterns were generated using 128 samples for each bill scan as comparedto the preferred use of 64 samples. FIG. 10A shows the correlationbetween a test pattern (represented by a heavy line) and a correspondingmaster pattern (represented by a thin line). It is clear from FIG. 10Athat the degree of correlation between the two patterns is relativelylow and exhibits a correlation factor of 606.

The manner in which the correlation between these patterns is increasedby employing progressive shifting is best illustrated by considering thecorrelation at the reference points designated as A-E along the axisdefining the number of samples. The effect on correlation produced by"single" progressive shifting is shown in FIG. 10B which shows "single"shifting of the test pattern of FIG. 10A. This is effected by dividingthe test pattern into two equal segments each comprising 64 samples. Thefirst segment is retained without any shift, whereas the second segmentis shifted by a factor of one data sample. Under these conditions, it isfound that the correlation factor at the reference points located in theshifted section, particularly at point E, is improved.

FIG. 10C shows the effect produced by "double" progressive shiftingwhereby sections of the test pattern are shifted in three stages. Thisis accomplished by dividing the overall pattern into three approximatelyequal sized sections. Section one is not shifted, section two is shiftedby one data sample (as in FIG. 10B), and section three is shifted by afactor of two data samples. With "double" shifting, it can be seen thatthe correlation factor at point E is further increased.

On a similar basis, FIG. 10D shows the effect on correlation produced by"triple" progressive shifting where the overall pattern is first dividedinto four (4) approximately equal sized sections. Subsequently, sectionone is retained without any shift, section two is shifted by one datasample, section three is shifted by two data samples, and section fouris shifted by three data samples. Under these conditions, thecorrelation factor at point E is seen to have increased again.

FIG. 10E shows the effect on correlation produced by "quadruple"shifting, where the pattern is first divided into five (5) approximatelyequal sized sections. The first four (4) sections are shifted inaccordance with the "triple" shifting approach of FIG. 10D, whereas thefifth section is shifted by a factor of four (4) data samples. From FIG.10E it is clear that the correlation at point E is increased almost tothe point of superimposition of the compared data samples.

The advantage of using the progressive shifting approach, as opposed tomerely shifting by a set amount of data samples across the overall testpattern, is that the improvement in correlation achieved in the initialsections of the pattern as a result of shifting is not neutralized oroffset by any subsequent shifts in the test pattern. It is apparent fromthe above figures that the degree of correlation for sample pointsfalling within the progressively shifted sections increasescorrespondingly.

More importantly, the progressive shifting realizes substantialincreases in the overall correlation factor resulting from patterncomparison. For instance, the original correlation factor of 606 (FIG.10A) is increased to 681 by the "single" shifting shown in FIG. 10B. The"double" shifting shown in FIG. 10C increases the correlation number to793, the "triple" shifting of FIG. 10D increases the correlation numberto 906, and, finally, the "quadruple" shifting shown in FIG. 10Eincreases the overall correlation number to 960. Using the aboveapproach, it has been determined that used currency bills which exhibita high degree of narrow dimension shrinkage and which cannot beaccurately identified as belonging to the correct currency denominationwhen the correlation is performed without any shifting, can beidentified with a high degree of certainty by using progressive shiftingapproach, preferably by adopting "triple" or "quadruple" shifting.

Referring now to FIG. 11, there is shown apparatus 210 for currencydiscrimination and counting which embodies the principles of the presentinvention. The apparatus comprises a housing 212 which includes left andright hand sidewalls 214 and 216, respectively, a rear wall 218, and atop surface generally designated as 220. The apparatus has a frontsection 222 which comprises a generally vertical forward section 224 anda forward sloping section 225 which includes side sections provided withcontrol panels 226A and 226B upon which various control switches foroperating the apparatus, as well as associated display means, aremounted.

For accepting a stack of currency bills 228 (FIG. 12) which have to bediscriminated according to denomination, an input bin 227 is defined onthe top surface 220 by a downwardly sloping support surface 229 on whichare provided a pair of vertically disposed side walls 230, 232 linkedtogether by a vertically disposed front wall 234. The walls 230, 232 and234, in combination with the sloping surface 229, define an enclosurewhere the stack of currency bills 228 is positioned.

From the input bin, currency bills are moved along a tri-sectionaltransport path which includes an input path where bills are moved alonga first direction in a substantially flat position, a curved guidewaywhere bills are accepted from the input path and guided in such a way asto change the direction of travel to a second different direction, andan output path where the bills are moved in a flat position along thesecond different direction across currency discrimination means locateddownstream of the curved guideway, as will be described in detail below.In accordance with the improved optical sensing and correlationtechnique of this invention, the transport path is defined in such a waythat currency bills are accepted, transported along the input path, thecurved guideway, and the output path, and stacked with the narrowdimension "W" of the bills being maintained parallel to the transportpath and the direction of movement at all times.

The forward sloping section 225 of the document handling apparatus 210includes a platform surface 235 centrally disposed between the sidewalls 214, 216 and is adapted to accept currency bills which have beenprocessed through the currency discrimination means for delivery to astacker plate 242 where the processed bills are stacked for subsequentremoval. More specifically, the platform 235 includes an associatedangular surface 236 and is provided with openings 237, 237A from whichflexible blades 238A, 240A of a corresponding pair of stacker wheels238, 240, respectively, extend outwardly. The stacker wheels aresupported for rotational movement about a stacker shaft 241 disposedabout the angular surface 236 and suspended across the side walls 214and 216. The flexible blades 238A, 240A of the stacker wheels cooperatewith the stacker platform 235 and the openings 237, 237A to pick upcurrency bills delivered thereto. The blades operate to subsequentlydeliver such bills to a stacker plate 242 which is linked to the angularsurface 236 and which also accommodates the stacker wheel openings andthe wheels projecting therefrom. During operation, a currency bill whichis delivered to the stacker platform 235 is picked up by the flexibleblades and becomes lodged between a pair of adjacent blades which, incombination, define a curved enclosure which decelerates a bill enteringtherein and serves as a means for supporting and transferring the billfrom the stacker platform 235 onto the stacker plate 242 as the stackerwheels rotate. The mechanical configuration of the stacker wheels andthe flexible blades provided thereupon, as well as the manner in whichthey cooperate with the stacker platform and the stacker plate, isconventional and, accordingly, is not described in detail herein.

The bill handling and counting apparatus 210 is provided with means forpicking up or "stripping" currency bills, one at a time, from bills thatare stacked in the input bin 227. In order to provide this strippingaction, a feed roller 246 is rotationally suspended about a drive shaft247 which, in turn, is supported across the side walls 214, 216. Thefeed roller 246 projects through a slot provided on the downwardlysloping surface 229 of the input bin 227 which defines the input pathand is in the form of an eccentric roller at least a part of theperiphery of which is provided with a relatively high friction-bearingsurface 246A. The surface 246A is adapted to engage the bottom bill ofthe bill stack 228 as the roller 246 rotates; this initiates theadvancement of the bottom bill along the feed direction represented bythe arrow 247B (see FIG. 13). The eccentric surface of the feed roller246 essentially "jogs" the bill stack once per revolution so as toagitate and loosen the bottom currency bill within the stack, therebyfacilitating the advancement of the bottom bill along the feeddirection.

The action of the feed roller 246 is supplemented by the provision of acapstan or drum 248 which is suspended for rotational movement about acapstan drive shaft 249 which, in turn, is supported across the sidewalls 214 and 216. Preferably, the capstan 248 comprises a centrallydisposed friction roller 248A having a smooth surface and formed of afriction-bearing material such as rubber or hard plastic. The frictionroller is sandwiched between a pair of capstan rollers 248B and 248C, atleast a part of the external peripheries of which are provided with ahigh friction-bearing surface 248D.

The friction surface 248D is akin to the friction surface 246A providedon the feed roller and permits the capstan rollers to frictionallyadvance the bottom bill along the feed direction. Preferably, therotational movement of the capstan 248 and the feed roller 246 issynchronized in such a way that the frictional surfaces provided on theperipheries of the capstan and the feed roller rotate in unison, therebyinducing complimentary frictional contact with the bottom bill of thebill stack 228.

In order to ensure active contact between the capstan 248 and a currencybill which is jogged by the feed roller 246 and is in the process ofbeing advanced frictionally by the capstan rollers 248B, 248C, a pair ofpicker rollers 252A, 252B, are provided for exerting a consistentdownward force onto the leading edges of the currency bills stationed inthe input bin 227. The picker rollers are supported on correspondingpicker arms 254A, 254B which, in turn, are supported for arcuatemovement about a support shaft 256 suspended across the side walls ofthe apparatus. The picker rollers are free wheeling about the arms andwhen there are no currency bills in contact with the capstan 248, beardown upon the friction roller 248A and, accordingly, are induced intocounter-rotation therewith. However, when currency bills are present andare in contact with the capstan 248, the picker rollers bear down intocontact with the leading edges of the currency bills and exert a directdownward force on the bills since the rotational movement of rollers isinhibited. The result is that the advancing action brought about bycontact between the friction-bearing surfaces 248D on the capstanrollers 248B, 248C is accentuated, thereby facilitating the strippingaway of a single currency bill at a time from the bill stack 228.

In between the picker arms 254A, 254B, the support shaft 256 alsosupports a separator arm 260 which carries at its end remote from theshaft a stationary stripper shoe 258 which is provided with a frictionalsurface which imparts a frictional drag upon bills onto which the pickerrollers bear down. The separator arm is mounted for arcuate movementabout the support shaft 256 and is spring loaded in such a way as tobear down with a selected amount of force onto the capstan.

In operation, the picker rollers rotate with the rotational movement ofthe friction roller 248A due to their free wheeling nature until theleading edges of one or more currency bills are encountered. At thatpoint, the rotational movement of the picker rollers stops and theleading edges of the bills are forced into positive contact with thefriction bearing surfaces on the periphery of the capstan rollers. Theeffect is to force the bottom bill away from the rest of the bills alongthe direction of rotation of the capstan. At the same time, theseparator shoe 258 also bears down on any of the bills that arepropelled forward by the capstan rollers.

The tension on the picker arm 254A is selected to be such that thedownward force exerted upon such a propelled bill allows only a singlebill to move forward. If two or more bills happen to be propelled out ofthe contact established between the picker rollers and the capstanrollers, the downward force exerted by the spring loaded shoe should besufficient to inhibit further forward movement of the bills. The tensionunder which the picker arm is spring loaded can be conveniently adjustedto control the downward bearing force exerted by the shoe in such a wayas to compliment the bill stripping action produced by the pickerrollers and the capstan rollers. Thus, the possibility that more thantwo bills may be propelled forward at the same time due to therotational movement of the capstan is significantly reduced.

The bill transport path includes a curved guideway 270 provided in frontof the capstan 248 for accepting currency bills that have been propelledforward along the input path defined by the forward section of thesloping surface 229 into frictional contact with the rotating capstan.The guideway 270 includes a curved section 272 which correspondssubstantially to the curved periphery of the capstan 248 so as tocompliment the impetus provided by the capstan rollers 248B, 248C to astripped currency bill.

A pair of idler rollers 262A, 262B is provided downstream of the pickerrollers for guiding bills propelled by the capstan 248 into the curvedguideway 270. More specifically, the idler rollers are mounted oncorresponding idler arms 264A, 264B which are mounted for arcuatemovement about an idler shaft 266 which, in turn, is supported acrossthe side walls of the apparatus. The idler arms are spring loaded on theidler shaft so that a selected downward force can be exerted through theidler rollers onto a stripped bill, thereby ensuring continued contactbetween the bill and the capstan 248 until the bill is guided into thecurved section 272 of the guideway 270.

A modifed feed mechanism is described in the assignee's copending U.S.patent application Ser. No. 07/680,585, filed Apr. 4, 1991, for "FeedArrangement For Currency Handling Machines," which is incorporatedherein by reference.

Downstream of the curved section 272, the bill transport path has anoutput path for currency bills. The output path is provided in the formof a flat section 274 along which bills which have been guided along thecurved guideway 270 by the idler rollers 262A, 262B are moved along adirection which is opposite to the direction along which bills are movedout of the input bin. The movement of bills along the direction ofrotation of the capstan, as induced by the picker rollers 252A, 252B andthe capstan rollers 248B, 248C, and the guidance provided by the section272 of the curved guideway 270 changes the direction of movement of thecurrency bills from the initial movement along the sloping surface 229of input bin 227 (see arrow 247B in FIG. 13) to a direction along theflat section 274 of the output path, as best illustrated in FIG. 13 bythe arrow 272B.

Thus, a currency bill which is stripped from the bill stack in the inputbin is initially moved along the input path under positive contactbetween the picker rollers 252A, 252B and the capstan rollers 248B,248C. Subsequently, the bill is guided through the curved guideway 270under positive contact with the idler rollers 262A, 262B onto the flatsection 274 of the output path.

In the output path, currency bills are positively guided along the flatsection 274 by means of a transport roller arrangement which includes apair of axially spaced, positively driven transport rollers 301, 302which are respectively disposed on transport shafts 303 and 304supported across the sidewalls of the apparatus. The first transportroller 301 includes a pair of projecting cylindrical sections 301A, 301Bwhich preferably have a high-friction outer surface, such as by theprovision of knurling thereupon. The second transport roller 302 whichis downstream of the first roller along the flat section of thetransport path also has similar cylindrical high-friction knurledsections 302A and 302B.

The flat section 274 is provided with openings through which each of theknurled sections of the transport rollers 301 and 302 are subjected tocounter-rotating contact with corresponding passive transport rollers305A, 305B, 306A and 306B. The passive rollers are mounted below theflat section 274 of the transport path in such a manner as to befreewheeling about their axes and biased into counter-rotating contactwith the corresponding knurled sections of the first and secondtransport rollers. While any appropriate mechanical suspending andpressuring arrangement may be used for this purpose, in the illustrativeembodiment passive rollers 305A and 306A are biased into contact withknurled sections 301A and 302B by means of an H-shaped leaf spring 307.The rollers are cradled in a freewheeling fashion within each of the twocradle sections of the spring through a support shaft (not shown)appropriately suspended about the spring. The arrangement is such thatthe leaft spring 307 is mounted relative to the passive rollers 305A and306A in such a way that a controllable amount of pressure is exertedagainst the rollers and pushes them against the active rollers 301 and302. A similar leaf spring/suspension arrangement is used to mount theother set of passive rollers 305B and 306B into spring-loaded,freewheeling counter-rotating contact with the knurled sections 301B and302B of the active transport rollers 301 and 302.

Preferably, the points of contact between the active and passive rollersare made coplanar with the output path so that currency bills can bemoved or positively guided along the path in a flat manner under thepositive contact of the opposingly disposed active and passive rollers.

The distance between the two active transport rollers and, of course,the corresponding counter-rotating passive rollers, is selected to bejust short of the length of the narrow dimension of the currency billsthat are to be discriminated. Accordingly, currency bills are firmlygripped under uniform pressure between the two sets of active andpassive rollers within the scanhead area, thereby minimizing thepossiblity of bill skew and enhancing the reliability of the overallscanning and recognition process.

The first active transport roller 301 is driven at a speed substantiallyhigher than that of the capstan rollers in the feed section. Since thepassive rollers are freewheeling and the active rollers are positivelydriven, the first transport roller 301 causes a bill that comes betweenthe roller and its corresponding passive rollers 305A, 305B along theflat section of the output path to be pulled into the nip formed betweenthe active and passive rollers (more specifically, between these passiverollers and the corresponding knurled sections 301A, 301B on the activetransport roller). The higher speed of the active transport rollerimparts an abrupt acceleration to the bill which strips the bill awayfrom any other bills that may have been guided into the curved guidewayalong with the particular bill being acted upon by the transport roller.

Currency bills are subsequently moved downstream of the first transportroller along the flat section into the nip formed between the knurledsections 302A, 302B on the second active transport roller 302 and thecorresponding passive rollers 306A, 306B with the second activetransport roller being driven at the same speed as that of the firsttransport roller.

The disposition of the second transport roller is selected to be suchthat the positive contact exerted by the cylindrical knurled sections302A, 302BA on the second transport roller 302 and the correspondingpassive rollers 306A, 306B upon a currency bill moving along the outputpath occurs before the bill is released from the similar positivecontact between the knurled sections 301A, 301B on the first transportroller 301 and the corresponding passive rollers 305A, 305B. As aresult, the second transport roller 302 and its corresponding passiverollers 306A, 306B together positively guide a currency bill through thescanhead area (where the transport rollers are located) onto the stackerplatform 235, from where the stacker wheels 238, 240 pick up the billand deposit it onto the stacker place 242.

Bills are held flat against the scanhead 18 by means of a plurality ofO-rings 308 which are disposed in corresponding grooves 309 on thetransport rollers 301 and 302. In a preferred arrangement, five suchO-rings 308A-E are used, one at each end of the transport rollers andthree in the central regions of the rollers.

The positive guiding arrangement described above is advantageous in thatuniform guiding pressure is maintained upon bills as they aretransported through the optical scanhead area; more importantly, this isrealized without adding significantly to mechanical complexity. Ineffect, the bill feeding operation is made stable, and twisting orskewing of currency bills is substantially reduced. This positive actionis supplemented by the use of the H-spring for uniformly biasing thepassive rollers into contact with the active rollers so that billtwisting or skew resulting from differential pressure applied to thebills along the transport path is avoided. The O-rings 308 function assimple, yet extremely effective means for ensuring that the bills areheld flat. Since the O-rings constitute standard off-the shelf items,any adjustment of the center distance between the two active transportrollers can be conveniently accommodated.

Referring now in particular to FIGS. 14 and 15, there are shown side andtop views, respectively, of the document processing apparatus of FIGS.11-13, which illustrate the mechanical arrangement for driving thevarious means for transporting currency bills along the three sectionsof the transport path, i.e., along the input path, the curved guidewayand the output path. As shown therein, a motor 320 is used to impartrotational movement to the capstan shaft 249 by means of a belt/pulleyarrangement comprising a pulley 321 provided on the capstan shaft 249and which is linked to a pulley 322 provided on the motor drive shaftthrough a belt 323. The diameter of the driver pulley 321 is selected tobe appropriately larger than that of the motor pulley 322 in order toachieve the desired speed reduction from the typically high speed atwhich the motor 320 operates.

The drive shaft 247 for the drive roller 246 is provided with rotarymotion by means of a pulley 324 provided thereupon which is linked to acorresponding pulley 321 provided on the capstan shaft 249 through abelt 326. The pulleys 324 and 321 are of the same diameter so that thedrive roller shaft 247 and, hence, the drive roller 246, rotate inunison with the capstan 248 mounted on the capstan shaft 249.

In order to impart rotational movement to the transport rollers, apulley 327 is mounted on the transport roller shaft 287 corresponding tothe first set of transport rollers and is linked to a correspondingpulley 328 on the capstan shaft 249 through a belt 329. The diameter ofthe transport roller pulley 327 is selected to be appropriately smallerthan that of the corresponding capstan pulley 328 so as to realize astepping-up in speed from the capstan rollers to the transport rollers.The second set of transport rollers mounted on the transport rollershaft 288 is driven at the same speed as the rollers on the first set oftransport rollers by means of a pulley 330 which is linked to thetransport pulley 327 by means of a belt 325.

As also shown in FIGS. 14 and 15, an optical encoder 299 is mounted onone of the transport roller shafts, preferably the passively driventransport shaft 288, for precisely tracking the lateral displacement ofbills supported by the transport rollers in terms of the rotationalmovement of the transport shafts, as discussed in detail above inconnection with the optical sensing and correlation technique of thisinvention.

In order to drive the stacker wheels 238 and 240, an intermediate pulley330 is mounted on suitable support means (not shown) and is linked to acorresponding pulley 331 provided on the capstan shaft 249 through abelt 332. Because of the time required for transporting currency billswhich have been stripped from the currency stack in the input binthrough the tri-sectional transport path and onto the stacker platform,the speed at which the stacker wheels can rotate for deliveringprocessed bills to the stacker plate is necessarily less than that ofthe capstan shaft. Accordingly, the diameter of the intermediate pulley333a is selected to be larger than that of the corresponding capstanpulley 331 so as to realize a reduction in speed. The intermediatepulley 333a has an associated pulley 333 which is linked to a stackerpulley 334 provided on the drive shaft 241 for the stacker wheels 238,240 by means of a belt 335. In the preferred embodiment shown in FIGS.11-15, the stacker wheels 238, 240 rotate in the same direction as thecapstan rollers. This is accomplished by arranging the belt 335 betweenthe pulleys 333, 334 in a "Figure-8" configuration about an anchoringpin 336 disposed between the two pulleys.

The curved section 272 of the guideway 270 is provided on its undersidewith an optical sensor arrangement 299, including an LED 298, forperforming standard currency handling operations such as counterfietdetection using conventional techniques, doubles detection, lengthdetection, skew detection, etc. However, unlike conventionalarrangements, currency discrimination according to denomination is notperformed in this area, for reasons described below.

According to a feature of this invention, optical scanning of currencybills, in accordance with the above-described improved optical sensingand correlation technique, is performed by means of an optical scanhead296 which is disposed downstream of the curved guideway 270 along theflat section 274 of the output path. More specifically, the scanhead 296is located under the flat section of the output path between the twosets of transport rollers. The advantage of this approach is thatoptical scanning is performed on bills when they are maintained in asubstantially flat position as a result of positive contact between thetwo sets of transport rollers at both ends of the bill along theirnarrow dimension.

It should be understood that the above-described drive arrangement isprovided for illustrative purposes only. Alternate arrangements forimparting the necessary rotational movement to generate movement ofcurrency bills along the tri-sectional transport path can be used justas effectively. It is important, however, that the surface speed ofcurrency bills across the two sets of transport rollers be greater thanthe surface speed of the bills across the capstan rollers in order toachieve optimum bill separation. It is this difference in speed thatgenerates the abrupt acceleration of currency bills as the bills comeinto contact with the first set of transport rollers.

The drive arrangement may also include a one-way clutch (not shown)provided on the capstan shaft and the capstan shafts, the transportroller shafts and the stacker wheel shafts may be fitted with fly-wheelarrangements (not shown). The combination of the one-way clutch and thefly wheels can be used to advantage in accelerated batch processing ofcurrency bills by ensuring that any bills remaining in the transportpath after currency discrimination are automatically pulled off thetransport path into the stacker plate as a result of the inertialdynamics of the fly wheel arrangements.

As described above, implementation of the optical sensing andcorrelation technique of this invention requires only a relatively lownumber of reflectance samples in order to adequately distinguish betweenseveral currency denominations. Thus, highly accurate discriminationbecomes possible even though currency bills are scanned along theirnarrow dimension. However, the accuracy with which a denomination isidentified is based on the degree of correlation between reflectancesamples on the test pattern and corresponding samples on the storedmaster patterns. Accordingly, it is important that currency bills betransported across the discrimination means in a flat position and, moreimportantly, at a uniform speed.

This is achieved in the bill handling apparatus of FIGS. 11-15, bypositioning the optical scanhead 296 on one side of the flat section 274of the output path between the two sets of transport rollers. In thisarea, currency bills are maintained in positive contact with the twosets of rollers, thereby ensuring that the bills move across thescanhead in a substantially flat fashion. Further, a uniform speed ofbill movement is maintained in this area because the second set ofpassive transport rollers is driven at a speed identical to that of theactive transport rollers by means of the drive connection between thetwo sets of rollers. Disposing the optical scanhead 296 in such afashion downstream of the curved guideway 270 along the flat section 274maintains a direct correspondence between reflectance samples obtainedby the optically scanning of bills to be discriminated and thecorresponding samples in the stored master patterns.

According to a preferred embodiment, the optical scanhead comprises aplurality of light sources acting in combination to uniformly illuminatelight strips of the desired dimension upon currency bills positioned onthe transport path below the scanhead. As illustrated in FIGS. 17-18,the scanhead 296 includes a pair of LEDs 340, 342, directing beams oflight 341A and 343B, respectively, onto the flat section 274 of theoutput path against which the scanhead is positioned. The LEDs 340, 342are angularly disposed relative to the vertical axis Y in such a waythat their respective light beams combine to illuminate the desiredlight strip. The scanhead 296 includes a photodetector 346 centrallydisposed on an axis normal to the illuminated strip for sensing thelight reflected off the strip. The photodetector 346 is linked to acentral processing unit (CPU) (not shown) for processing the sensed datain accordance with the above-described principles of this invention.Preferably, the beams of light 340A, 340B from the LEDs 340, 342,respectively, are passed through an optical mask 345 in order to realizethe illuminated strips of the desired dimensions.

In order to capture reflectance samples with high accuracy, it isimportant that the photodetector capture reflectance data uniformlyacross the illuminated strip. In other words, when the photodetector 346is positioned on an axis passing through the center of the illuminatedstrip, the illumination by the LEDs as a function of the distance fromthe central point "0" along the X axis, should optimally approximate astep function as illustrated by the curve A in FIG. 19. With the use ofa single light source angularly displaced relative to the vertical, thevariation in illumination by an LED typically approximates a Gaussianfunction, as illustrated by the curve B in FIG. 19.

In accordance with a preferred embodiment, the two LEDs 340 and 342 areangularly disposed relative to the vertical axis by angles α and β,respectively. The angles α and β are selected to be such that theresultant strip illumination by the LEDs is as close as possible to theoptimum distribution curve A in FIG. 19. According to a preferredembodiment, the angles α and β are each selected to be 19.9 degrees. TheLED illumination distribution realized by this arrangement isillustrated by the curve designated as "C" in FIG. 19 which effectivelymerges the individual Gaussian distributions of each light source toyield a composite distribution which sufficiently approximates theoptimum curve A.

The manner in which the plurality of light strips of differentdimensions are generated by the optical scanhead by means of an opticalmask is illustrated in FIG. 16-18. As shown therein, the optical mask345 essentially comprises a generally opaque area in which two slits 354and 356 are formed to allow light from the light sources to pass throughso as to illuminate light strips of the desired dimensions. Morespecifically, slit 354 corresponds to the wide strip used for obtainingthe reflectance samples which correspond to the characteristic patternfor a test bill. In a preferred embodiment, the wide slit 354 has alength of about 0.500" and a width of about 0.050". The second slit 356forms a relatively narrow illuminated strip used for detecting the thinborderline surrounding the printed indicia on currency bills, asdescribed above in detail. In a preferred embodiment, the narrow slit356 has a length of about 0.300" and a width of about 0.010".

It is preferred that a separate pair of light sources 340 and 342 beprovided for each of the two slits 354 and 356. Thus, as can be seen inFIGS. 17 and 18, a first pair of LEDS 340A and 342A are provided for thenarrow slit, and a second pair of LEDs 340B and 342B are provided forthe second slit. Similarly, two separate photodetectors 346A and 346Bare provided for detecting reflected light from the two slits. As can beseen in FIGS. 17 and 18, the channel for transmitting reflected lightfrom the narrow slit to the photodetector 346A is narrower in thetransverse direction than the channel for transmitting reflected lightfrom the wide slit to the photodetector 346B.

According to another feature of the present invention, the undesireddoubling or overlapping of bills in the transport system is detected bythe provision of a pair of optical sensors which are co-linearlydisposed opposite to each other within the scan head area along a linethat is perpendicular to the direction of bill flow, i.e., parallel tothe edge of test bills along their wide dimensions as the bills aretransported across the optical scan head. As best illustrated in FIG.20, the pair of optical sensors S1 and S2 (having corresponding lightsources and photodetectors which are not shown here) are co-linearlydisposed within the scan head area in close parallelism with the widedimension edges of incoming test bills. In effect, the optical sensorsS1 and S2 are disposed opposite each other along a line within the scanhead area which is perpendicular to the direction of bill flow.

It should be noted that FIGS. 11, 13 and 15 also include an illustrationof the physical disposition of the sensors S1 and S2 within the opticalscanhead area of the currency recognition and counting apparatus. Forpurposes of clarity, the sensors S1 and S2 are represented only in theform of blocks which correspond to the light sources associated with thesensors. Although not illustrated in the drawings, it should be notedthat corresponding photodetectors (not shown) are provided within thescanhead area in immediate opposition to the corresponding light sourcesand underneath the flat section of the transport path. These detectorsdetect the beam of coherent light directed downwardly onto the billtransport path from the light sources corresponding to the sensors S1and S2 and generate an analog output which corresponds to the sensedlight. Each such output is converted into a digital signal by aconventional ADC convertor unit (not shown) whose output is fed as adigital input to and processed by the system CPU (not shown), in amanner similar to that indicated in the arrangement of FIG. 1.

The presence of a bill which passes under the sensors S1 and S2 causes achange in the intensity of the detected light, and the correspondingchange in the analog output of the detectors serves as a convenientmeans for density-based measurements for detecting the presence of"doubles" (two or more overlaid or overlapped bills) during the currencyrecognition and counting process. For instance, the sensors may be usedto collect a predefined number of density measurements on a test bill,and the average density value for a bill may be compared topredetermined density thresholds (based, for instance, on standardizeddensity readings for master bills) to determine the presence of overlaidbills or doubles.

A routine for using the outputs of the two sensors S1 and S2 to detectany doubling or overlapping of bills is illustrated in FIG. 21. Thisroutine starts when the demonination of a scanned bill has beendetermined at step 401, as described previously. To permit variations inthe sensitivity of the density measurement, a "density setting choice"is retrieved from memory at step 402. The operator makes this choicemanually, according to whether the bills being scanned are new bills,requiring only a high degree of sensitivity, or used bills, requiring alower level of sensitivity. After the "density setting choice" has beenretrieved, the system then proceeds through a series of steps whichestablish a density comparison value according to the denomination ofthe bill. Thus, step 403 determines whether the bill has been identifiedas a $20 -bill, and if the answer is affirmative, the $20-bill densitycomparison value is retrieved from memory at step 404. A negative answerat step 403 advances the system to step 405 to determine whether thebill has been identified as a $100-bill, and if the answer isaffirmative, the $100-bill density comparison value is retrieved frommemory at step 406. A negative answer at step 405 advances the system tostep 407 where a general density comparison value, for all remainingbill denominations, is retrieved from memory.

At step 408, the density comparison value retrieved at step 404, 406 or407 is compared to the average density represented by the output ofsensor S1. The result of this comparison is evaluated at step 409 todetermine whether the output of sensor S1 identifies a doubling of billsfor the particular denomination of bill determined at step 401. If theanswer is negative, the system returns to the main program. If theanswer is affirmative, step 410 then compares the retrieved densitycomparison value to the average density represented by the output of thesecond sensor S2. The result of this comparison is evaluated at step 401to determine whether the output of sensor S2 identifies a doubling ofbills. Affirmative answers at both step 409 and step 411 results in thesetting of a "doubles error" flag at step 412, and the system thenreturns to the main program. The "doubles error" flag can, of course, beused to stop the bill transport motor.

FIG. 22 illustrates a routine that enables the system to detect billswhich have been badly defaced by dark marks such as ink blotches,felt-tip pen marks and the like. Such sever defacing of a bill canresult in such distorted scan data that the data can be interpreted toindicate the wrong denomination for the bill. Consequently, it isdesirable to detect such severely defaced bills and then stop the billtransport mechanism so that the bill in question can be examined by theoperator.

The routine of FIG. 22 retrieves each successive data sample at step 450and then advances to step 451 to determine whether that sample is toodark. As described above, the output voltage from the photodetector 26decreases as the darkness of the scanned area increases. Thus, the lowerthe output voltage from the photodetector, the darker the scanned area.For the evaluation carried out at step 451, a preselected thresholdlevel for the photodetector output voltage, such as a threshold level ofabout 1 volt, is used to designate a sample that is "too dark."

An affirmative answer at step 451 advances the system to step 452 wherea "bad sample" count is incremented by one. A single sample that is toodark is not enough to designate the bill as seriously defaced. Thus, the"bad sample" count is used to determine when a preselected number ofconsecutive samples, e.g., ten consecutive samples, are determined to betoo dark. From step 452, the system advances to step 453 to determinewhether ten consecutive bad samples have been received. If the answer isaffirmative, the system advances to step 454 where an error flag is set.This represents a "no call" condition, which causes the bill transportsystem to be stopped in the same manner discussed above in connectionwith FIG. 8A.

When a negative response is obtained at step 451, the system advances tostep 455 where the "bad sample" count is reset to zero, so that countalways represents the number of consecutive bad samples received. Fromstep 455 the system advances to step 456 which determines when all thesamples for a given bill have been checked. As long as step 456 yields anegative answer, the system continues to retrieve successive samples atstep 450. When an affirmative answer is produced at step 456, the systemreturns to the main program at step 457.

It is desirable to maintain a predetermined space between each pair ofsuccessive bills to facilitate the resetting of the scanning systembetween the trailing edge of the scanned area on one bill and theleading borderline on the next bill. The routine for performing thisspacing check is illustrated in FIG. 23. This routine begins with step500, which checks the output signals from the sensors S1 and S2 todetermine when the leading edge of a bill is detected by either sensor.The detection of a predetermined change in the output from either sensorS1 or S2 advances the system to step 501, which determines whether thedetected output change is from the first sensor to see the leading edgeof a bill. If the answer is affirmative the system returns to the mainprogram at step 503. A negative response at step 501 advances the systemto step 504 to determine whether the spacing check is done yet. If theanswer is "yes," the system returns to the main program. If the answeris "no," step 505 determines whether a spacing check is to be performed,based on whether the first bill in a new stack of bills placed in theCRU has been detected. That is, there is no need to initiate a spacingcheck until the first bill reaches the sensors S1 and S2. Thus, anegative answer at step 505 returns the system to the main program,while an affirmative answer advances the system to step 506 whichcompares the actual spacing count, i.e., the number of encoder pulsesproduced after detection of the leading edge of the bill, to apreselected minimum spacing count retrieved from memory. If the actualspacing count is above the preselected minimum, there is no error andconsequently the next step 507 yields a negative response, indicatingthat there is no spacing error. Thi negative response sets a "spacingerror checked" flag at step 509. If the actual spacing count is belowthe preselected minimum, step 509 detects a spacing error andconsequently produces an affirmative response which sets an error flagat step 508. The system then returns to the main program at step 503. Itis this flag that is read at step 504.

A routine for automatically monitoring and making any necessarycorrections in various line voltages is illustrated in FIG. 24. Thisroutine is useful in automatically compensating for voltage drifts dueto temperature changes, aging of components and the like. The routinestarts at step 550 which reads the output of a line sensor which ismonitoring a selected voltage. Step 551 determines whether the readingis below 0.60, and if the answer is affirmative, step 552 determineswhether the reading is above 0.40. If step 552 also produces anaffirmative response, the voltage is within the required range and thusthe system returns to the main program step 553. If step 551 produces anegative response, an incremental correction is made at step 554 toreduce the voltage in an attempt to return it to the desired range.Similarly, if a negative response is obtained at step 552, anincremental correction is made at step 555 to increase the voltagetoward the desired range.

We claim:
 1. A currency counting and evaluation device for receiving astack of currency bills, rapidly counting and evaluating all the billsin the stack, and then re-stacking the bills, said device comprisingafeed mechanism for receiving a stack of currency bills and feeding saidbills in the direction of the narrow dimension of the bills, one at atime, to a feed station, a bill transport mechanism for transportingbills, in the direction of the narrow dimension of the bills, from saidfeed station to a stacking station, at a rate in excess of about 800bills per minute, a stationary optical scanning head located betweensaid feed and stacking stations for scanning a preselected segment of acentral portion of each bill transported between said stations by saidtransport mechanism, said scanning head including at least one lightsource for illuminating a strip of said preselected segment of a bill,and at least one detector for receiving reflected light from theilluminated strip on the bill and producing an output signalrepresenting variations in the intensity of the reflected light, meansfor sampling said output signal at preselected intervals as a bill ismoved across said scanning head in the direction of the narrow dimensionof the bill, each of said output signal samples being proportional tothe intensity of the light reflected from a different strip of saidpreselected segment of a bill, a memory for storing characteristicsignal samples produced by scanning said preselected segments of billsof different denominations with said scanning head and sampling saidoutput signal at said preselected intervals, each of said stored signalsamples being proportional to the intensity of the light reflected froma different strip of said preselected segment of a bill, and signalprocessing means for receiving said signal samples and (1) determiningthe denomination of each scanned bill by comparing said stored signalsamples with said output signal samples produced by the scanning of eachbill with said scanning head, (2) counting the number of scanned billsof each denomination, and (3) accumulating the cumulative value of thescanned bills of each denomination.
 2. The currency counting andevaluation device of claim 1 which includes an encoder coupled to saidtransport mechanism for monitoring the movement of each bill byproducing a repetitive tracking signal synchronized with incrementalmovements of said transport mechanism, and means urging each bill intofirm engagement with said transport mechanism and said scanning head toensure a fixed relationship between the increments of movement of eachbill and the corresponding increments of movement of said transportmechanism which is synchronized with said encoder.
 3. The currencycounting and evaluation device of claim 1 wherein said transportmechanism is driven at a controllable speed.
 4. The currency countingand evaluation device of claim 1 wherein said detector in said scanninghead is a single photodetector which produces an electrical outputsignal proportional to the intensity of the light reflected from thescanned bill.
 5. The currency counting and evaluation device of claim 4which includes means for sampling said output signal at incrementssynchronized with said repetitive tracking signal, and at the sameincrements used in said characteristic signals stored in said memory. 6.The currency counting and evaluation device of claim 1 wherein saidstrips are dimensioned so that at least 50 different strips can bescanned in the direction of the narrow dimension of each bill.
 7. Thecurrency counting and evaluation device of claim 1 which includes meansfor detecting a borderline around the image printed on each bill, andwherein said preselected segment is located inside said borderline, andthe scanning of said preselected segment is initiated at a prescribedinterval following the detection of said borderline.
 8. The currencycounting and evaluation device of claim 1 wherein said preselectedsegment of each bill is located in the central region of the bill. 9.The currency counting and evaluation device of claim 1 wherein saidfeeding and stacking stations are both located at the front of saiddevice, and said transport mechanism carries bills rearwardly away fromsaid feed station and then returns the bills forwardly to said stackingstation.
 10. The currency counting and evaluation device of claim 9wherein said transport mechanism forms a linear path for said bills onthe upstream side of said stacking station, and said scanning head islocated along said linear path.
 11. The currency counting and evaluationdevice of claim 1 wherein said preselected segment of each bill isscanned in less than one tenth of a second.
 12. The currency countingand evaluation device of claim 1 wherein said light source illuminatessaid preselected segment of each bill from opposite sides of saiddetector.
 13. The currency counting and evaluation device of claim 1which includes means for controlling the movement of a selected billbetween said scanning head and the stacking station for that bill inresponse to said determination of the denomination of that bill.
 14. Thecurrency counting and evaluation device of claim 13 wherein saidcontrolling means stops the movement of said selected bill between saidscanning head and the stacking station for that bill.
 15. The currencycounting and evaluation device of claim 13 wherein said controllingmeans decelerates said selected bill before stopping the movement ofthat bill.
 16. The currency counting and evaluation device of claim 13wherein the denomination of each bill is determined before the leadingedge of that bill reaches the stacking station for that bill.
 17. Thecurrency counting and evaluation device of claim 13 wherein saidstacking station is spaced from said scanning head by a distance that isless than the width of two of said bills.
 18. The currency counting andevaluation device of claim 1 which includes signal processing meansresponsive to the output signals from said detector for determining thedenomination of each scanned bill before that bill has been advanced toa stacking station, andmeans responsive to said signal processing meansfor altering the movement of a scanned bill in response to thedenomination determination for that bill, before that bill is advancedto a stacking station.